Diffractive optical element, objective optical system, optical pick-up device and optical information recording reproducing apparatus

Abstract

A diffractive optical element, comprises a first diffractive structure to generate n11-th order diffracted ray when a first light flux having λ1 (nm) of a wavelength comes in and to generate n21-th order diffracted ray (n11.≧n21) when a second light flux having λ2 (nm) (λ2>λ1) comes in; and a second diffractive structure to generate n12-th order diffracted ray when the first light flux comes in and to generate n22-th order diffracted ray (n12.≧n22) when the second light comes in. The diffractive optical element satisfies the following formula: δφA12≠δφB12, where δφA12={n11·λ1 / (N11−1)} / {n21·λ2 / (N21−1)}, and δφB12={n12·λ1 / (N12−1)} / {n22·λ2 / (N22−1)}, wherein N11, N21 are refractive index of the first diffractive structure respectively for λ1 and λ2, and N12, N22 are refractive index of the second diffractive structure respectively for λ1 and λ2.

Description

BACKGROUND OF THE INVENTION [0001] The present invention relates to a diffractive optical element, objective optical system, optical pick-up device and optical information recording reproducing apparatus, and particularly, to an optical pick-up device and optical information recording reproducing apparatus by which, by using light fluxes projected from light sources of different wavelengths, the information can be recorded and / or reproduced for optical information recording media, and to a diffractive optical element and objective optical system appropriate when used for them. [0002] Conventionally, an optical pick-up device which can record / reproduce for a plurality of kinds of optical disks having different recording densities, is well-known. For example, there is a device by which a DVD (Digital Versatile Dik) and a CD (Compact Disk) are recorded / reproduced by using an optical pick-up device. Further, recently, as an optical disk having different recording densities, an optical p...

AI Technical Summary

Benefits of technology

[0011] Then, in the diffractive optical element according to the present invention, a second diffractive structure in which a ratio δφB12 of the optical path length of the wavelength λ1 and wavelength λ2 is different from the ratio δφA12 of the optical path length in the above first diffractive structure, is provided. By using the difference between the diffraction actions of the first diffractive structure and the second diffractive structure, the wavelength dependency of the spherical aberration of the first diffractive structure in the blue violet wavelength range can be controlled, or suppressed small. Hereby, an increase of a yield in the mass production of the blue violet laser light source, and a reduction of the production cost of the blue violet laser light source or optical pick-up device become possible. Further, even when the temperature is changed, a stable recording / reproducing of the information on the high density optical disk can also be conducted.

Problems solved by technology

However, in the diffractive sructure which generates the diffraction light of the combination of the diffraction order n1 and the diffraction order n2, because a changing amount of the spherical aberration per unit wavelength becomes large, the selection of the laser light source becomes necessary, and there are a problem which results in, an increase of the production cost of the laser light source, and an increase of the production cost of the optical pick-up device, and a problem that, because the spherical aberration of the objective optical system largely changes by the wavelength change of the laser light source following the temperature change, the stable recording / reproducing characteristic for the high density optical disk can not be obtained.

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